EVIDENCE OF WIDESPREAD DEWATERING IN THE HOLOCENE SEDIMENTS OF WESTERN LAKE SUPERIOR

High resolution acoustic data collected in Lake Superior reveals that the lake floor is not a featureless basin, rather that it possesses fine-scale features. These are the "fingerprints" of processes that have shaped the development of the basin. Some of these are no longer active while others continue today.

Some of the most interesting lake floor features observed in Lake Superior are the ring shaped depressions that are widely developed in areas below wave-base, where fine-grained sediments are preserved. These features are typically 100-300m across, 25-50m wide and up to 5m deep. They occur both as interconnected clusters and also as isolated structures. Multibeam imagery reveals that rings are composed of chains of pockmarks.

High-resolution seismic data, collected with a 28 kHz echosounder, show that the lake floor pockmarks are underlain by a layer bound system of small extensional faults that are widely developed in the very fine grained glacio-lacustrine clays. They exhibit a polygonal plan form geometry with a large range of strikes and oblique to orthogonal intersection geometries. This system is interpreted to be an immature polygonal fault system (PFS) that developed in the very fine grained glacio-lacustrine clays sometime after deposition. PFS formation, driven by the volumetric contraction of the sedimentary pile, led to the release of a significant quantity of pore fluid creating the widespread lake floor pockmarks.